The subject invention relates to radiolabeling dopamine receptor ligands. More specifically, the subject invention relates to radioiodination of the dopamine receptor ligand generally known as spiperone, and certain derivatives thereof.
It has been known for some time that spiperone is an antagonist of the dopamine receptor. Further, tritiated spiperone has long been used as a research tool for the characterization and quantitation of dopamine receptors, particularly in the study of certain disease states which result in quantitative changes in dopaminergic receptors such as Parkinson's and Huntington's diseases, and schizophrenia. Tritium, however, is a weak beta emitter which requires quantitation by liquid scintillation counting. Tritium is also a low specific activity isotope, therefore assays being tritiated spiperone require inconveniently large amounts of tissue.
Radiobrominated spiperone has also been prepared for use in vitro to test dopamine receptor binding affinity, and in vivo to image dopamine receptors and to study stimulation of prolactin secretion. Use of radiobrominated spiperone, however, suffers from the disadvantage of having a very short half-life (57 hours). Also, Br-77 is not commercially available.
Preparation of non-radioactive iodinated spiperone has also been reported. However, this non-radioactive iodinated spiperone was reported to have a twenty-fold decrease in dopamine receptor binding affinity. Also, the process used for this iodination is unknown.